any_cast std::any pointer to specific pointer type

问题

I would like std::map<string, any> *var1 to point to the same memory address/value as std::any *var2 because I know that var2 is pointing to a map<string, any>.

The following seems to work:

std::map<string, any> *var1 = any_cast<std::map<string, any>>(var2);

Is it ok? The problem is that it does not signal bad cast even if var2 is actually not an std::map<string, any>* but something else, but if it is it still works.

Am I doing it right?

std::map<string, any> *mapptr;
std::any realmap = std::map<string, any>({{"key", "value"}, {"key2", 5}});
std::any *ptrtomap = &realmap;
mapptr = any_cast<std::map<string, any>>(ptrtomap);
// Interesting part below
realmap = 6;
mapptr = any_cast<std::map<string, any>>(ptrtomap);

After changing the type of the variable to integer the any pointer is pointing to I can still cast that pointer to a pointer of type std::map.

What???

回答1:

Try running this code

#include <stdio.h>
#include <map>
#include <string>
#include <any>

using namespace std;

int main()
{
    map<string, any> *amap;
    any realmap = std::map<string, any>({{"key", "value"}, {"key2", 5}});
    any *ptrtomap = &realmap;
    amap = any_cast<map<string, any>>(ptrtomap);
    printf("Pointer real map: %x\n", amap);

    // Interesting part below
    realmap = 6;
    amap = any_cast<map<string, any>>(ptrtomap);
    printf("Pointer invalid map: %x\n", amap);
    return 0;
}

it outputs e.g.

Pointer real map: 11cfd00
Pointer invalid map: 0

So the illegal conversion returned a null pointer. The compiler can't detect invalid casts

回答2:

The problem is that it does not signal bad cast even if var2 is actually not an std::map* but something else

As reported in the documentation of std::any (overload n.5):

If operand is not a null pointer, and the typeid of the requested T matches that of the contents of operand, a pointer to the value contained by operand, otherwise a null pointer.

So actually you can check the cast's successful just checking the pointer.

mapptr = std::any_cast<std::map<string, any>>(&realmap);
if (!mapptr) {
  // invalid cast conversion
}

回答3:

Just like dynamic_cast, any_casting a pointer returns the null pointer when it fails - it does not throw an exception.

来源：https://stackoverflow.com/questions/57429709/any-cast-stdany-pointer-to-specific-pointer-type

标签

c++

pointers

casting

any